Wihelm

You must fight Wilhelm in the 'A Train to Catch' story mission. You are lured to a train that supposedly contains the Vault Key, but instead contains this robot. Wilhelm is one of Handsome Jack's most powerful and trusted minions.

This can be a tricky fight if you do not know what you are doing. Weapons that deal explosive damage work especially well against him, as they do with most robots in the game. Fire damage is resisted. Completing Tina's "You are Cordially Invited" quest chain gives you the corrosive Teapot pistol that should make Wilhelm a pushover. Make sure to kill any and all surveyors that you see before they have a chance to repair him. Strafe constantly and don't let him get too close and you should have little trouble defeating Wilhelm.

There is also a location specific challenge that requires you to kill Wilhelm without killing any of his surveyors. Completing this challenge awards you 5 badass ranks.